The present invention relates generally to external-cavity, surface-emitting semiconductor lasers. It relates in particular to an external-cavity, surface-emitting semiconductor laser that is tunable in discrete equally-spaced frequency steps corresponding to frequencies of optical channels in an optical communication system.
In optical communications systems, information is transmitted along an optical fiber as a modulated beam of light. In one preferred optical communication arrangement, wavelengths for the light beam are in a range between about 1515 nanometers (nm) to about 1565 nm, corresponding to a frequency range from about 198,000 gigahertz (GHz) to about 190,000 GHz. In a scheme referred to as dense wavelength-division-multiplexing (DWDM) the frequency range is partitioned into 40 channels at 100 GHz intervals. A trunk optical fiber may carry up to 40 different beams at 40 different wavelengths, one corresponding to each channel. The different-wavelengths (optical-carrier) beams are generated by InGaAsP diode-lasers, one for each channel. The output of each laser is modulated to encode the information to be transmitted onto the laser-beam provided by the laser. Communications channels are separated from or added to the trunk optical fiber by wavelength-selective couplers.
In order to accommodate increasing optical communications traffic DWDM systems that will include channels spaced apart by 50 GHz and eventually 25 GHz are being developed.
In current optical communications systems, conventional edge-emitting diode-lasers are typically used to provide the laser beam. These diode-lasers are arranged to deliver light in a single longitudinal mode (single frequency). In the manufacture of diode-lasers it extremely difficult, if not impossible to provide that the output frequency of a diode-laser corresponds with sufficient precision to the frequency of a particular optical communications channel. In order to overcome this difficulty the lasers are usually arranged to be continuously tunable (usually by temperature tuning) to the required channel frequency. Temperature tuning is sufficiently accurate and stable that for current systems with 100 GHz channel spacing a diode-laser tuned to a particular channel frequency can be retained at that frequency purely by maintaining the temperature of the diode-laser with about xc2x11.0xc2x0 C. of the tuning temperature of the diode-laser.
In a system with 50 GHz or 25 GHz channel spacing, simple temperature control measures alone would be insufficient to retain an edge emitting diode-laser tuned to a particular channel frequency. In this case it would be necessary to provide a closed-loop arrangement for locking the output frequency to a frequency standard such as an etalon or a fiber Bragg grating. By way of example such an arrangement may involve generating an error signal corresponding to a variation of the laser output frequency from the channel frequency, and using the error signal to adjust the temperature of the diode-laser to restore the output frequency to the channel frequency. Controlling 80 diode-lasers (one per channel) in this way would add to the cost and complexity of the DWDM system. Accordingly, there is a need for a laser that can be passively retained at a channel frequency in a 50 GHz spaced DWDM system.
The present invention is directed to a laser than can be tuned to any selected one of a plurality of equally-spaced frequencies. Such a plurality of equally-spaced frequencies, for example, can be frequencies corresponding to channel frequencies in a DWDM optical communications system.
In one aspect, the inventive laser comprises, a laser resonator terminated by first and second mirrors. A surface-emitting semiconductor multilayer gain-structure is located in said laser resonator in optical contact with the first mirror. A pumping arrangement is provided for energizing the multilayer gain-structure and causing laser radiation to be generated in the laser resonator. The laser resonator is configured such that the laser radiation can be generated at any time in only one of a plurality of possible longitudinal oscillating modes. The first and second mirrors are spaced apart by an optical distance selected such that the frequency and frequency-spacing of the possible longitudinal lasing modes correspond with the plurality of equally-spaced frequencies. An optical filter is located in the laser resonator. The optical filter is tunable for tuning the peak transmission frequency thereof. The first optical filter has a bandwidth arranged such that when said peak transmission frequency of said first optical filter is tuned to a value about equal to the selected one of the plurality of equally-spaced frequencies, the laser resonator delivers radiation in a single longitudinal mode only at the selected one of the plurality of equally-spaced frequencies.
As the frequency of the laser operating modes is determined by the spacing of the first and second mirrors these frequencies can be fixed to the extent that the distance between the first and second mirrors can be fixed. One preferred separation distance for the first and second mirrors is about 6.0 millimeters (mm). This mirror spacing provides a frequency spacing of 25 GHz. If the first and second mirrors are fixed on an aluminum base a temperature variation of xc2x11.0xc2x0 C. will cause a frequency variation of only 0.6 GHz. This frequency variation is within the channel width of DWDM communication system.
In a preferred embodiment of the inventive laser, the first optical filter is an etalon. A third mirror is located in said laser resonator and spaced-apart from the first mirror and the gain-structure. The etalon is formed by the first and third mirrors, and has a peak transmission frequency determined by the optical distance between the first and third mirrors. The third mirror is selectively movable with respect to the first mirror for tuning the peak transmission frequency of the etalon.
In another preferred embodiment the first and third mirrors are spaced apart by a distance that, absent any measure to the contrary, would provide a frequency spacing of a first series of possible longitudinal oscillating modes that was a sub multiple of a desired frequency spacing, with only a subset thereof corresponding to the plurality of equally-spaced frequencies. In order to reduce the possible oscillating modes to correspond with the plurality of equally-spaced frequencies a second optical filter is included in the laser resonator. The second optical filter has a plurality of transmission peaks that correspond to the subset of the first set of oscillating modes and has a bandwidth (of transmission peaks) selected to suppress oscillation at all other longitudinal oscillating modes.
Preferably the second optical filter is an etalon. The etalon is preferably formed by supporting the third mirror on one of first and second opposite surfaces of a transparent substrate. The etalon is formed between the third mirror and the other of the first and second opposite surfaces of the substrate. The optical thickness of the substrate is selected such that transmission peaks of the etalon correspond to the plurality of equally-spaced frequencies.